The consequences of spinal cord injury and subsequent recovery of function in newborn and adult rats are not uniform. The observation that the locomotor patterns and motor strategies which recover after spinal cord injury differ between newborn and adult operates suggests that the mechanisms underlying this recovery also must differ. The long range goal of this research program is to define the mechanisms underlying recovery of function following spinal cord injury and to identify methods to enhance that recovery. During the previous period of support we have demonstrated that transplants of fetal spinal cord tissue (homotypic transplants) mediate recovery of function after spinal cord injury in newborn and adult rats. This recovery of function is dependent upon anatomical integration of the transplant with the host CNS and the formation of connections between host and transplant. We have also demonstrated, however, that non-target (heterotypic) transplants are also capable of modifying behavioral recovery. The experiments in the current proposal will test the hypothesis that transplants of fetal spinal cord tissue mediate recovery of function both by specific mechanisms (i.e. permitting the regeneration of particular descending and segmental pathways) and by non-specific mechanisms (such as trophic effects). The studies proposed will determine the contribution of anatomical reorganization to recovery of function after spinal cord lesions in newborn and adult rats. We hypothesize that segmental, intersegmental and suprasegmental pathways make unique contributions to anatomical plasticity and recovery of function after spinal cord lesions and transplants and that their relative contribution differs following lesions in the adult versus lesion in the neonate. Studies are also proposed to test the hypothesis that the administration of exogenous trophic support will increase both the extent of regeneration and the degree of recovery of function after spinal cord injury and transplantation in neonatal and adult operates. We will continue to use our model system of spinal cord injury and transplantation in newborn and adult rats. We will examine the response to injury of 4 representative pathways, which differ in their relative stages of their development at birth and determine their contribution to recovery of function. The studies proposed use sensitive qualitative and quantitative behavioral analysis of specific tests of reflex and locomotor function, and qualitative and quantitative neuroanatomical tracing techniques to determine the mechanisms which underlie recovery of function after spinal cord injury in the developing and mature CNS and to identify ways to enhance that recovery.